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Thursday, June 21, 2012

Hey, You Little Weasel!

This is the first in a
series of articles on some of the ten members of the weasel family that are
found in Michigan.

An adult least weasel (Mustela nivalis) weighs just 1-2
ounces and can fit its supple 6-8 inch-long body into a coffee cup.It doesn’t spend much time in water.A river otter (Lutia canadensis) can reach
nearly 5 feet in length and weigh 30 pounds, and is a terrific swimmer, easily catching
fish and other prey underwater.The two
animals might seem much different, but both are members of the weasel
(mustelid) family and have many common traits.And like several other close relatives, they are poorly understood.

Michigan is home to ten members (see sidebar) of the weasel family.As a group, mustelids are known for their
fierceness, big appetites, and several unique adaptations.Mustelids have characteristic anal glands
used both for defense and for marking territory.And many members of the weasel family can postpone
pregnancies until long after mating.In
the larger weasels, like wolverines, this “delayed implantation” can postpone
births for years.This can help
reproduction when harsh environments are often unsuitable for raising young.

The least weasel is found throughout Michigan but is not
common anywhere.Even experienced naturalists
seldom see least weasels.The animals turn
white in winter in the Upper Peninsula and northern Lower Peninsula, but some
individuals stay brown in winter in the southern part of the state.The least weasel is active both day and night
and in all seasons, searching almost constantly for prey over home ranges of just
an acre or two.It may have several dens
it regularly visits, and will also temporarily use abandoned burrows of other
small animals.

The least weasel’s primary prey is meadow voles, mouse-like
mammals which moves in distinct runways in grassy areas.However, like other weasels, it will eat
anything it can kill.Mice, birds, eggs,
and insects are all eaten regularly, and excess food (including prey parts) are
often stored (cached) in the dens.The
least weasel’s appetite is legendary, as it consumes almost half of its own
body weight daily.

Two larger “cousins” of the least weasel are also found in
our state.Michigan is near the southern
edge of the range of the short-tailed weasel (Mustela erminea), also called the
ermine or stout.Adults weigh 2 to 6
ounces (males are twice the size of females) and are 7 to 13 inches long.Michigan is near the northern edge of the
long-tailed weasel’s (Mustela frenata) range.That species tips the scales at 3 to 9 ounces and is 11 to 22 inches in
length.The three species can usually be
distinguished fairly easily if captured as the short-tailed weasel has white
feet year-round.The least weasel is not
only smaller than the other two, but has an even shorter tail that lacks a
black tip.All three species can be
confused if merely glimpsed while moving in and out of cover.

Of course, the short-tailed and long-tailed weasels take
much larger prey than does the least weasel, and they generate far more tales
of hunting and fighting prowess.They
occasionally kill prey much larger than themselves such as young rabbits, rats,
and snakes.The long-tailed weasel will
also take small woodchucks and even baby pigs, and has a reputation as a
chicken killer.If a weasel can get on
the back of an animal, wrap its legs around it, and make a bite at the base of
the prey’s skull deep enough to reach the brain or spinal cord, it can and will
kill it.

“Surplus killing” of more animals than it can eat is common
among all of the weasels.This behavior
is an adaptation crucial to survival in winter and other times when prey can
become scarce.Weasels have such high
energy demands that fasting for more than a day or so is detrimental.A weasel’s slim body lets it follow prey
through burrows and other small openings, but the high surface-to-volume ratio
works against it in winter.So, weasels
can’t simply curl-up to stay warm and wait-out cold weather (like raccoons, for
example).They must have stock-piled
food or find prey quickly.(If humans
had the same per pound energy requirements as weasels, a person would have to
eat more than 50 pounds of food a day.)Not surprisingly, starvation in winter is a major cause of weasel
mortality.

Weasels are killed by foxes, coyotes, cats, hawks, owls and
many other predators.Camouflage and
hiding are the most important defenses, but individual weasels are sometimes
able to put up amazing fights when attacked.A naturalist in Europe reported seeing a hawk fly off with a weasel only
to come crashing lifeless to the ground after the still-alive weasel worked its
way free enough to deliver a mid-air killing bite to the hawk.There are many reports of weasels using their
quickness to fight off dogs; however, cats can usually kill small weasels
without too much trouble.

The delayed implantation exhibited by short-tailed and
long-tailed weasels and some other mustelids is of much interest to
biologists.After mating, the resultant
fertilized eggs don’t implant themselves in the uterus as occurs in most other
mammals.The eggs stay free but are
somehow protected until biochemical clues, poorly understood by scientists,
trigger implantation.Then, the “true
pregnancy” is fairly short (less than a month in the smaller weasels).Many scientists are puzzled because it would
seem that animals that eat so often and don’t live very long would get no
advantage from delayed implantation.Indeed, delayed implantation has been gradually lost in the least weasel
and certain other mustelids as they evolved.Least weasel reproduction is more reactionary.They will have two or three litters of
(usually four or five) young a year if prey is abundant (but just one if prey
is scarce.)In the weasels in which the
adaptation persists, it is likely that delayed implantation allows the females
to be ready to breed at all times.If
males are not always available that helps optimize timing of births.

Also poorly understood are the unique vocalizations of
weasels.These include shrill shrieks
made not only when the animals feel threatened, but also sometimes when they
spot prey.In the latter situation, it
might seem more advantageous to launch a silent attack.But apparently, a shrill shriek may cause
prey to “freeze in position” and might make them easier to catch.

2 comments:

Just saw what looks like a least weasel here in Grand Haven. It had killed a vole and was dragging it under our deck. I couldn't believe it. He/she looked right at me and proceeded with its task.It seemed almost smaller than its prey.

I just saw a least weasel here in Grand Haven. It had killed a vole and was dragging it under our wooden deck. It looked right at me and did not seem afraid. It was amazing. I saw its little eyes and white underbelly. The vole was bigger than the weasel.